The invention relates to a continuous process for preparing oil in water emulsions and, more particularly, to a process for controlling the particle size of the emulsion within a comparatively narrow particle size range. The invention includes as one embodiment a process for preparing microcapsules characterized by having a substantially uniform size distribution. The latter process yields microcapsules having a median particle size (diameter) of about 4 to 25 microns which are suitable for use in making carbonless papers.
For purposes of this application, the median microcapsule size is on a volume basis. That is, 50% of the volume of hydrophobic core material is contained within microcapsules below the median microcapsule size and 50% of the material is contained within microcapsules above the median microcapsule size.
In a preferred embodiment, the hydrophobic core material contains a chromogenic material dissolved therein and paper coated with these microcapsules can be used as a self-contained or transfer carbonless copy paper. The chromogenic material may be a color precursor, color developer or color former.
U.S. Pat. No. 3,565,817 issued on Feb. 23, 1971 to Lissant discloses a method of preparation of emulsions in which the volume of internal phase is greater than that of the external phase. Lissant made his high internal phase ratio emulsions continuously by (1) preparing a preformed emulsion of approximately the same character, (2) introducing into the preformed emulsion both internal and external phases while maintaining the emulsified mass in a state of continuous shear and (3) withdrawing the prepared emulsion at the desired rate. Care was exercised not to maintain the shear above the shear stability point of the emulsion. Thus in Lissant's method he could not use colloid mills and similar high shear devices. No attempt was made to produce solid walled microcapsules.
U.S. Pat. No., 3,812,057 issued May 21, 1974 to de la Torriente et al. and U.S. Pat. No. 3,816,331 issued Jun. 14, 1974 to Brown et al. disclose continuous processes and apparatus therefor for preparing the microcapsules by coacervation by pumping the necessary ingredients for encapsulation through a conduit in turbulent flow. The ingredients may be introduced into the manufacturing conduit in the form of a premix of all ingredients as in de la Torriente et al. or at separate stages as in Brown et al.
Processes and apparatus which can be operated continuously to produce microcapsules are disclosed in U.S. Pat. No. 3,015,128 to Somerville; U.S. Pat. No. 3,160,686 to Doyle et al.; U.S. Pat. No. 3,202,533 to; Sachel et al.; U.S. Pat. No. 3,310,612 to Somerville; U.S. Pat. No. 3,423,489 to Arens et al.; U.S. Pat. No. 4,218,409 to Dannelly.